A growing need for maritime awareness has prompted significant research on technologies to allow for better monitoring of vessels. The systems range from automated transponders to Synthetic Aperture Radar (SAR) and Electro-Optical (EO) remote sensors, coupled with sophisticated processing algorithms. Part of the monitoring involves monitoring of the vessel speeds, and significant remote sensing research has been devoted to processing algorithms that can derive information from vessel wakes in SAR and EO images. In spite of the progress in these areas, key limitations remain. In particular, wake algorithms can only become useful once they are seamlessly integrated into a general image analysis system that allows a user to view, geo-reference, enhance and interpret the imagery at will. Mainly for this reason, a practical system has never been available that would allow a user to infer speed from wakes quickly and efficiently from a single remote collection.
It is currently possible to monitor vessel speeds in the vicinity of major ports and harbors using the Automatic Identification System (AIS). AIS transponders are required aboard all passenger-carrying ships and all international voyaging ships over 300 gross tons, primarily for collision avoidance. The transponders broadcast electronic messages once per minute to other ships and shore receivers, messages that include speed data from the navigation instruments. Clear drawbacks are that AIS transponders are not universally required, the transponders can possibly fail, and ship crews might decide to operate them in non-compliant ways.
Alternatively, high altitude remote sensing provides an independent means of speed measurement. SAR systems, e.g. RADARSAT-2, can be used to directly detect ships and measure their speeds via Doppler frequency shift measurements. However, researchers acknowledge several difficulties in determining ship speed from a single SAR collection pass, e.g. the need to identify fixed scattering objects, and the complicating effect of ship pitch and roll motions. In addition, target objects require relatively high Radar Cross Section (RCS) compared to the RCS variance of the surrounding natural sea surface. Small vessels can have negligible RCS, in particular those made from nonconductive materials such as wood or fiberglass. Finally, for both SAR and EO sensors, spatial resolution is a common limiting factor for direct monitoring.
An alternative way to infer a vessel's speed is by remotely observing the detailed structure of its wave wake, or its “Kelvin wake”, a familiar phenomenon named for Lord Kelvin (William Thomson), who was credited with its first clear description in 1891. Both the Kelvin wake and the distinct “turbulent wake” commonly extend many kilometers away from the vessel, in both SAR and EO images. The classic Kelvin wake is a steady-state pattern of transverse and divergent surface gravity wave systems generated by the disturbance of the moving vessel. In sufficiently deep water, i.e. if the “Depth Froude Number” (DFN) is nearly zero, the pattern is bound by two rays, each approximately 19.5° from the ship's line of direction of motion. On these rays the two wave systems constructively interfere to form prominent “cusp waves”, making the distinctive “V” shape. As early as 1881 Edmund Froude had already discovered a simple relationship between a ship's speed and the wavelength of its transverse wave. While today it is sometimes possible to apply this by measuring the transverse wave from high altitude imagery, the rapid decay of this wave usually makes it difficult. In contrast, the cusp waves decay more slowly, and the wavelength measured along the cusp line on either side of the “V” is also a simple function of vessel speed.
In view of the above, it is an object of the present invention to provide an overhead imagery system that allows a user to infer the speeds of vessels through the water, via the extraction of wake features in the images. Another object of the present invention is to provide an overhead imagery system and methods that determine a vessels speed through passive optical imagery taken from a sufficiently high altitude over the water surface. Still another object of the present invention is to provide an overhead imagery system and methods therefor that function as an unobtrusive “plug-in” module within the user's existing image viewing computer software system. Yet another object of the present invention is to provide an overhead imagery system and methods therefor that work even if only one passive optical image can be acquired, and even if the actual vessels are too small to be discerned in the image, i.e. only the vessel wake can be seen in the image. Still another object of the present invention is to provide an overhead imagery system and methods therefor that can be leveraged to improve maritime intelligence gathering and law enforcement. Another object of the present invention is to provide an overhead imagery system that extracts vessel speed information quickly, efficiently and accurately from remotely sensed imagery, to allow monitoring vessel speeds of many vessels and over large areas. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.